In recent years, from social demands for low-fuel consumption and low exhaust emission, there has been an increased attention to an electric vehicle or a hybrid vehicle which has an alternate-current (AC) motor to run. For example, in a hybrid vehicle, an AC motor is connected to a direct-current (DC) power source such as a rechargeable battery unit through a power converter such as an inverter. The inverter converts a DC voltage supplied from the DC power source to an AC voltage and drives the AC motor with the AC voltage.
A typical control system for controlling an AC motor mounted on an electric vehicle or a hybrid vehicle uses two or three current sensors to detect two or three of three phases of the AC motor. The AC motor is controlled based on outputs (i.e., current detection values) of the current sensors. In this type of motor control system, two or three current sensors are provided to one AC motor. Therefore, the size and cost of the control system may be increased.
A method of reducing the number of current sensors used to control an AC motor has been known. For example, in JP-A-2004-64903, one current sensor is provided to a DC power line to detect a bus current. However, if a current sensor is provided to a DC power line in a control system for controlling a high-powered motor mounted on a vehicle, conduction noise may be increased due to an extension of the DC power line. JP-A-10-225199, JP-A-2001-145398 corresponding to U.S. Pat. No. 6,229,719, JP-A-2004-159391, and JP-A-2008-86139 corresponding to US 2008/0079385 disclose a method of controlling an AC motor by using a current sensor that detects an electric current flowing through one of three phases of the AC motor.
In JP-A-10-225199, a d-axis current Id and a q-axis current Iq are calculated from a state equation, which is derived from a motor constant, using a current detection value of one of three phases of an AC motor. However, the motor constant changes with a temperature. Therefore, the d-axis current Id and the q-axis current Iq, which are calculated from the state equation, may have large estimation errors so that control may become unstable. Further, since the calculation is complex, it may be difficult to cause an electronic control unit (ECU) to perform the calculation.
In JP-A-2001-145398, a d-axis current Id and a q-axis current Iq of an AC motor are calculated by dq transformation of a current detection value of one of three phases of the AC motor and current estimation values of the other phases. Then, averaging is performed through a first order lag filter, and then inverse dq transformation is performed. However, a lag may occur in the current estimation values due to the first order lag filter so that control may become unstable.
In JP-A-2004-159391, an electric current flowing through one (e.g., U-phase) of three phases of an AC motor is detected by a current sensor. Then, current estimation values of the other phases (e.g., V-phase and W-phase) are calculated based on a current detection value detected by the current sensor, a d-axis current command value, a q-axis current command value, and an electrical angle of the AC motor.
Specifically, a U-phase current phase angle θ1 (=α+θ) is calculated by adding a command current phase angle α, which is formed by a q-axis and a combined vector of a d-axis current command value Id* and a q-axis current command value Iq*, to an angle θ, which is formed by an U-phase axis of a stator and a rotor of the AC motor. Then, a current amplitude Ia is calculated from a formula (A) using the U-phase current phase angle θ1 and a current detection value Iu detected by the current sensor. Then, current estimation values Iv and Iw of the other phases are calculated from the respective formulas (B) and (C) using the current amplitude Ia and the U-phase current phase angle θ1.Ia=Iu/{√{square root over (⅓)}×[−sin(θ1)]}  (A)Iv=√{square root over (⅓)}×Ia×[−sin(θ1+120°)]  (B)Iw=√{square root over (⅓)}×Ia×[−sin(θ1+240°)]  (C)
A d-axis current Id and a q-axis current Iq are calculated based on the current detection value Iu and the current estimation values Iv and Iw. Then, the current flowing through the AC motor is feedback-controlled by calculating a voltage command value of the AC motor so that the d-axis current Id and the q-axis current Iq can be equal to the d-axis current command value Id* and the d-axis current command value Iq*, respectively.
It is noted that a current vector of an AC motor changes with a command current vector (current vector corresponding to a current command value). However, due to effects of the control error and the feedback control, there is a deviation (i.e., difference) between the current vector and the command current vector. Accordingly, a command current phase differs from an actual current phase and does not actually reflect the actual current phase.
However, according to JP-A-2004-159391, by using the U-phase current phase angle calculated from the command current phase angle, the current estimation values of the other phases are calculated without consideration of the actual current phase. As a result, there is a possibility that the current estimation values do not have accuracy high enough to control the AC motor.
In JP-A-2008-86139, an electric current flowing through one (e.g., U-phase) of three phases of an AC motor is detected by a current sensor. Then, current estimation values of the other phases (e.g., V-phase and W-phase) are calculated based on a current detection value detected by the current sensor and three-phase current command values of the AC motor.
Specifically, the AC motor is controlled by dq transformation using an electrical angle based on a current detection value Iu detected by the current sensor and three-phase electric command values Iv* and Iw* of the other phases. The electric command values Iv* and Iw* are calculated by transformation using an electrical angle based on a d-axis current command value Id* and a d-axis current command value Iq* of the AC motor. However, since the electric command values are simply used as a substitute for current detection values of the other phases, a command current phase does not actually reflect an actual current phase. Therefore, in a vehicle, in which a torque and a rotation speed are required to change from time to time, estimated currents will not be calculated sufficiently accurately, and hence the AC motor will not be controlled properly.
As described above, there remains a technical difficulty in controlling a high-powered AC motor by using one current sensor.